This java applet is a quantum mechanics simulation that shows
electric dipole transitions, which are
interactions of classical electromagnetic radiation with
an atom. It demonstrates absorption and stimulated emission.

When the applet starts up, there is an incoming wave which is at
the right frequency (the Bohr frequency)
to transition the particle from the ground state to 2px. Once
it reaches that state, it transitions back
to the ground state, and so on.

To view another transition, make a selection from the popup menu
in the upper-right.